Field of the Invention
The present invention relates to a light diffusion sheet for overlaying, and a backlight unit.
Discussion of the Background
Liquid crystal display devices are intensively used as flat panel displays taking advantages of characteristic features such as thin modeling, weight saving and low electric power consumption, and use thereof has been increasing over time, such as televisions, personal computers, mobile phone terminals such as smartphones, portable information terminals such as tablet terminals, and the like.
As such liquid crystal display devices, those of a backlight system in which a liquid crystal panel is irradiated from the back face side are in widespread use, which are equipped with a backlight unit of an edge-lit type (side-lit type), an immediate beneath type or the like. The edge-lit backlight unit 101 provided in such a liquid crystal display device include, as shown in FIG. 5 in general: a light source 102; a rectangular plate-shaped optical waveguide plate 103 disposed to be aligned with the light source 102 along the ends thereof; a plurality of optical sheets 104 provided to overlay the front face side of the optical waveguide plate 103; and a reflection sheet 105 provided on a back face side of the optical waveguide plate 103. The optical waveguide plate 103 is generally made from a synthetic resin, in which a polycarbonate, acrylic resin or the like is used as a principal component. As the light source 102, LED (light emitting diode), a cold-cathode tube or the like is used, and in light of reduction in size, energy saving and the like, LED is in widespread use for the present. In addition, the optical sheet 104 which may be used includes: (1) a light diffusion sheet for underlaying 106 superposed on a front face side of the optical waveguide plate 103, and predominantly has a light diffusion function; (2) a prism sheet 107 superposed on a front face side of the light diffusion sheet for underlaying 106, and has a refracting function toward a normal direction side; and (3) a light diffusion sheet for overlaying 108, superposed on a front face side of the prism sheet 107, and inhibits lack in uniformity of the luminance resulting from the shape and the like of prism portions of the prism sheet 107 through slightly diffusing the rays of light (see Japanese Unexamined Patent Application, Publication No. 2005-77448). In addition, generally used light diffusion sheets for overlaying are provided with a substrate layer, and a light diffusion layer that is overlaid on the front face side of the substrate layer and has a resin matrix and resin beads.